It's called the Swopper. It's bouncy and sway-y, and it makes you sit with your back straight and your legs at approximately 135° to your torso, like so:
Now, I have a little experience with a variant of this posture, from sitting on a meditation cushion in a previous life, and I was sort of skeptical that it would be comfy for any length of time, but then I went and sat on one of these babies, in the flesh.
Sold.
Well, almost. They cost $600 (USD) retail. A small price to pay to keep my spine from disintegrating, but ouch. So I thought I'd surf around and see if I could find a screaming internet deal. The final price I'll end up paying lies somewhere near the intersection of my back pain and my parsimony. Stay tuned...